The present invention relates to a semiconductor device, more particularly, to an electric contact between a resistive diffusion layer formed in a semiconductor substrate and a conductive metal layer formed thereon.
In an integrated semiconductor circuit (IC), each resistor member is realized as a resistive diffusion layer formed in the semiconductor substrate, especially, as a diffusion layer having one conductivity type in a semiconductor substrate having the other conductivity type. The resistor member is then connected to the conductive metal layer formed on the semiconductor substrate, via the electric contact, so as to cooperate with another corresponding electric functional member or members. In such a case, the resistance value (R) is mainly determined by the resistance value (R.sub.d) of the resistive diffusion layer and, secondarily, by the ohmic contact resistance value (R.sub.c) created at the electrical contact. Thus, the actual resistance value (R) is usually defined as being a sum of the above-mentioned two resistance values (R.sub.d) and (R.sub.c). Accordingly, if the resistance value (R) must be strictly set to a predetermined value, not only the resistance value (R.sub.d) but also the resistance value (R.sub.c) must be strictly set so as to obtain the exact desired resistance value (R).
Generally, certain characteristics are known in the art regarding the resistance values (R.sub.d) and (R.sub.c). First, it is relatively easy to obtain a resistive diffusion layer which can maintain a correct resistance value (R.sub.d) for a very long time. Second, it is considerably difficult to obtain an electric contact which can maintain a correct resistance value (R.sub.c) for a very long time. The reason for this, briefly, is that, over a long period of time, undesired diffusion occurs in the area close to the electric contact when Si (silicon) atoms, which are in the diffusion layer, move into gaps between Al (aluminum) atoms which are in the conductive metal layer. Consequently, it is usually impossible to set exactly the resistance value (R) a correct value for a long time. For example, it is usually impossible to make resistor members for constructing a so-called current mirror circuit, because this type of circuit requires exactly correct resistance values, which must remain constant for a long period for time, of the respective resistor members.